Does speaker sensitivity change over time?

[lancethibault]

This post may not start out too technical, but if numbers start getting thrown around I just wanted to make sure the thread was in the right place.

As a speaker gets older does it become less sensitive?

Also, specifically, do the Dean G crossover mods to RF-7's change the sensitivity? If so, to what?

I have yet to see a post from someone who didn't prefer the mod on these particular speakers and everyone seems to point out that it make the high end more tame or less bright. So I'm just wondering if the cross over mod had some impact on the sensitivity.

thanks

[Coytee]

As a speaker gets older does it become less sensitive?

Not if it's anything like my wife.... [^o)] [:|] [:#]

[JL Sargent]

In time all things change. Anything man made changes even faster. Boy, thats kinda deep.

[russ69]

The simple answer is not enough to worry about.

Thanx, Russ

[WMcD]

I think the general answer is no, the sensitivity does not degrade.

All other things aside, like horn loading, the sensitivity is set by the motor strength. Or Bl (bee - el). The l is the length of wire in the voice coil. And we will assume all of that is in the magnetic gap.

The B is the strength of the magnetic field in the gap. This is established by the strenght of the permanent magnet in the driver. (Technically, there are structures which direct the field, too, the pole pieces.)

So, the issue becomes whether the permanent magnets lose their strenght.

In the old days, magnets were made of Alnico. Aluminum, nickel, and cobalt. These had some tendency to lose strenght. They can be remagnitized.

The wars in the Congo shut down cobalt mines there and so the favored material became a ferrite (iron based) slurry which was made into a solid piece. The slurry aspect led to them being called mud magnets. The -M in some designations. I think this is the same stuff used on magnetic recording tape and floppy disks.

See: http://en.wikipedia.org/wiki/Ferrite_(magnet)

You'll see that this is called a ceramic. Therefore you will see reference to ceramic magnets in drivers.

This is OT, but some of us remember double density versus high density floppy disks. The improved ones kept their recorded magnetic field better, but required a stronger writing field. You had to be careful.

In any event, the modern ferrite based magnets in speakers don't degrade, to speak of.

- - - -

As far as crossovers, these are filters. Changing the values will change what freqs get though to the driver. So they are in that way changing the sensitivity of the system at various freqs. .

Wm McD

[Deang]

Nice post Gil.

The RF-7 mod knocks off about 1.5dB between 3 and 4kHz. The overall response has peaks in other areas that are higher than the area I'm bringing down. I think the mod sounds good to people because it's hitting that area where our ears are fairly sensitive. At any rate, I'm not bringing the entire output of the horn down, just a small part of it. The short answer is no, sensitivity stays the same.

[lancethibault]

I really appreciate the info guys.

I think I'm going to pick some RF-7's up that already have the mod. Since I won't get a chance to actually hear them, I'm just trying to get an idea of what I might expect to hear. I know this is next to impossible, but I figured I could at least learn something if I start asking questions.

[WMcD]

I should have mentioned that there is regulating mechanism to the design.

The magnet is typically donut shaped. The pole pieces which direct or conduct the field to the ring-shaped gap in which the voice coil sits.

According to what I read . . . The pole pieces have a limit to the amount of magnetic field which they can conduct. They saturate based on the material used and their cross section.

Therefore, the scheme is to use a magnet which is somewhat larger than is necessary to saturate the pole pieces. In that way, variations in the strenght of the magnet do not affect the strenght of the field in the gap. That is, of course, unless the magnet becomes so weak that it no longer can saturate the pole pieces.

Wm McD

[JL Sargent]

Hmmm. and I thought one of the mechanisms for a speaker sounding bad with age was a loss of sensitivity. In affect the speaker is not able to do what it once had done. Is that not a loss of sensitivity? Seems like hardening of diaphrams, weaker magnets, old capacitors in crossovers, etc. would all contribute to this condition. To ask it another way: Can a 98db 1964 Klipsch speaker still produce the same 98db in 2009 without some help along the way?

[WMcD]

Those were the days of alnico. So maybe this is true on the motor stength issue..

Gee this reminds me of my professional work. The plaintiff's expert on the other side has armchair opinions about things that could happen to make things go bad (admittedly with some basis in theory -- as yours). Smile. There are some decisions by judges going to whether such evidence is admissible. They ask: "What is it here that makes your scenario more valid than one which says to the contrary?"

Heck, isn't that an attitude we should have with tweeks? For example: Proponents say that green magic marker on the edge of a CD helps the optical read be more accurate. But should we not suspect that the green magic marker actually degrades optical read? No data has ever been presented. [some edits here.]

So let me point out a few things.

I don't know why surronds would stiffen. The woofer in many Klipsch products are pleated paper. You'd think they would get more floppy with the working. I'm not sure about the stuff of midrange and tweeter diaphragms. But why assume they do?

I would not expect the woofer crossovers to go bad. In those days they only used inductors.

Bob C is pretty much the resident expert on caps. I'll defer to him. People report the replacing ones which are decades old leads to improved results. OTOH, I've not seen this reported in terms of output from the speaker.

The 1965 date is interesting. Reasonably, we don't have data on the forum where people have made technical tests.

I went to motor stenght as an answer for the reason that this is the primary index of sensitivity.

The issues of crossovers are generally descibed as whether they are lossy or perhaps have altered filtered characteristics. This is what Dean was talking to.

Alterations in suspension chacteristics would go to resonance freqs. They could become more lossy in that they could become more resistive. Again, I've not seen any reason to think they become more lossy, rather than less.

Wm McD

[JL Sargent]

Its an interesting discussion. It certainly seems plauseable that diaphrams harden over time and change characteristics but I have no data to support it.

Woofers could become more sensitive for all I know.

We didn't even touch on speaker coil reactance, as I'm guessing that could change. Don't know though.

I wonder what the sensitivity of a 60s speaker is in 2009 vs its original spec. I would be surpised if it hadn't lost a step.

Thanks for bouncing it around. Always enjoy your thoughts and comments.